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Influenza (or flu) is a highly contagious viral infection of the horse's respiratory system, similar to the flu in humans. What distinguishes influenza infection from the "common cold" (Rhinovirus) in horses, is that influenza attacks the entire length of the respiratory system (nose, throat, trachea, bronchial tubes), and cold viruses attack only the nose and throat. The consequences of having your entire respiratory tract infected are much more severe, because there is more widespread damage, impairment of airflow, coughing, vulnerability of bacterial infections, and more pronounced bodily reaction (chills, muscle soreness) to this heavy burden.
As with the similar disease in human beings, influenza is a highly contagious infection. It moves down a line of horses within 2 or 3 days, at most a week. Most horses are able to fight the influenza viral attack and recover within 2 weeks. However, a proportion of infected horses are unable to recover quickly from influenza, and they remain symptomatic for several weeks. Much less commonly, a minority of affected horses develop other problems such as pneumonia and cardiac irregularities. Antibiotics are frequently given to thwart off the possibility of a secondary bacterial infection, but in many cases they are not necessary. It depends on the severity of symptoms, the density and stress of horses in the barn.
Influenza is a common cause of illness in young athletic horses in training. These young horses are placed in yards and stables, which can be densely packed, and there can be movement of horses in and out. A racing or training stable is a good example. Younger horses have not developed good immunity against influenza, and usually succumb readily.
What to Watch For
Following influenza infection, a horse requires a lot of rest, because it is such a widespread infection of the respiratory system and body. Undoubtedly, if the horse does not get rest, he will suffer the consequences down the line. Interruption of training is costly in terms of wasted time at the training stable and delay of time before the horse is able to generate income. However, failure to allow sufficient rest during viral respiratory infections is an important cause of potentially severe and permanent and respiratory dysfunction. Unscrupulous and inexperienced trainers often treat affected horses with drugs intended to mask signs of infection so that the horse can continue (prematurely) in the training program or even race.
The influenza virus is widespread in horse populations. The main reasons influenza cannot be completely controlled in the horse industry include the following:
Other key reasons are that the virus is capable of altering its "appearance" to the horse's immune system and new strains of the virus evolve which are not recognized by the horse's immune system.
Because of the evasive nature of the influenza virus, it is necessary to administer influenza vaccine to horses at risk on a rather frequent basis (every 2 to 4 months). Frequent administration of influenza vaccine is especially important for young athletes in training, which are often being exposed to other young horses of uncertain vaccinal background. Vaccine manufacturers attempt to prevent horses from being blindsided by new strains of influenza virus by monitoring the influenza strains running through populations of horses, and incorporating those common strains in their vaccines.
Most horses are routinely vaccinated against influenza. Recent work has shown that vaccinating a young horse too soon may not be very effective because the antibodies acquired from the mare typically persist until the horse is 8 months of age. Vaccine against influenza should be given (by injection) to the horse (every 4 weeks for 3 initiating treatments) at nine, ten, and eleven months of age (substantially later in life than had been previously recommended!).
Alternatively, an intranasal vaccine against influenza might be considered and administered at 7 and 11 months of age. This new intranasal vaccine against equine influenza is both safe and effective. However, it is not certain how long any of these vaccines last, and if a new strain comes along, they will offer only partial protection. Partial protection is a lot better than full blown illness, that will cause weeks to months of recovery and hundreds of dollars to treat, so it's best to vaccinate.